The present invention relates to a semiconductor device having an IC (Integrated Circuit) chip and a cap hermetically sealing the IC chip.
Semiconductor devices of the kind described include one disclosed in IEICE TRANSACTIONS, VOL. E74, NO. 8 August 1991, page 2333, FIG. 2. The semiconductor device taught in this document has a TAB (Tape Automated Bonding) IC chip provided with TAB leads. The IC chip is packaged on a circuit board face down via a buffer implemented by silicone rubber and is connected to the board via the TAB leads. A cap is seam-welded to the circuit board along the edges of the open end thereof and is bonded to the top of the chip at the bottom or underside thereof, thereby hermetically sealing the chip. The silicone rubber intervening between the chip and the circuit board absorbs irregularity in the height and inclination of the chip when the chip is packaged on the circuit board. Specifically, the silicone rubber maintains the thickness of bond between the cap and the chip constant. However, when it comes to flip chip packaging for producing signals from the entire surface of an IC chip via bumps or micropin packaging for doing so via input/output micropins, it is not practicable to protect the bonding portion by silicon rubber or similar elastic material from below the chip. The flip chip packaging scheme and micropin packaging scheme are essential in coping with the increasing number of signals associated with an IC chip.
To eliminate the above problem, the IC chip may be bonded to solder bumps provided on a thin film formed on the circuit board and hermetically sealed by the cap, as proposed in the 41st ECTC Papers, 1991, page 704, FIG. 1. To absorb the irregularity in the configuration of the cap and the irregularity in the packaging height of the chip, a cap having prescribed dimensions is selected after the packaging of the chip, or the thickness of the solder is changed. Therefore, this type of semiconductor device is not practicable unless different kinds of caps are prepared beforehand, resulting in the increase in cost and the decrease in yield.
Further, when the irregularity in the height of the semiconductor device and the irregularity in the packaging height of the chip are not adjusted, there also occurs irregularities in the thickness of the adhesive for bonding the chip and cap, i.e., the adhesive intervening between the top of the chip and the bottom of the cap. For example, as the adhesive becomes thick, the thermal resistance from the chip or heat source to the top of the cap, i.e., a heat radiation surface increases to degrade the cooling effect. Moreover, the increase in the thickness of the adhesive makes it difficult to remove voids from the adhesive. This also increases the thermal resistance and causes cracks to occur due to thermal stresses, degrading the reliability and quality. In addition, when the cap and circuit board are soldered to each other, the solder is apt to fly apart onto the signal leads and circuit board in the event of connection. It is likely that this part of the solder melts due to heat in the event of repair to short-circuit the portions where the chip and circuit board are connected.